Many receivers require detection means that enable the receiver to detect a preamble or certain delimiters. This is of particular importance in communication systems where the transmitter and receiver operate in an asynchronous mode. Since the receiver does not know when to expect a signal with payload (herein also referred to as signal burst), the payload is typically preceded by a preamble or start delimiter that is detectable by the receiver.
Especially Bluetooth communication systems, where the preamble phase is very short (only 4 bits), require at the beginning of the signal burst a very fast settling procedure for the receiver. In Bluetooth applications, information is transmitted in the form of packets. A Bluetooth packet has an access code with a four bit preamble, a 64 bit “sync word”, a four bit trailer. This access code precedes the random payload data (plus header). The “sync word” is unique to the wireless connection that involves the receiving device. That is, a receiving device understands whether or not a packet received at its antenna is being sent to the receiving device (or another receiving device) by correlating (via a correlation circuit within the receiver's downstream processing circuitry) the sync word against the connection's unique word.
The received signal usually shows some frequency offset that makes it difficult for the receiver to reliably demodulate the package including the sync word. This implies that in some microseconds the frequency offset should be at least roughly removed prior to the sync word and at the same time the edges of the symbol clock signal should be roughly settled close to the middle of the received symbols.
In order to remove quickly the frequency offset in a Bluetooth GFSK modulated antenna signal, usually a MaxMin DC extraction circuit is implemented. In addition to this MaxMin detection, typically a leakage mechanism is employed to reduce the effect of the noise.
Known preamble detection schemes, such as the MaxMin detection scheme as used in conventional analog Bluetooth demodulators, are not very well suited for a digital implementation. Some kinds of preamble detection schemes require that the receiver is provided with a special triggering signal that indicates the signal burst. The respective receiver architectures are complicated.
It is important that the preceding noise does not degrade the required preamble settling process. The MaxMin algorithm, as used in prior art Bluetooth receivers, usually fails due to the preceding noise. Failing means here, that the package error rate (PER) is too high. If there are too many bit errors in the access code, then the received package is rejected which is considered to be a package error.
The location of the searched preamble sequence (e.g., “1010”) is especially aggravated by a low S/N ratio, if one assumes a low antenna input signal being received. In this case, the noise of the receiver front-end degrades the wanted signal.
It is a further disadvantage of known Bluetooth approaches based on the MaxMin algorithm, that the compensation may depend on potential wrong historical values. Due to this, the frequency demodulated signal after activating a conventional Bluetooth receiver, but prior to the signal burst, would generate a wrong initial value for the MaxMin algorithm and even the use of the known leakage concept either slows down the offset calculation or reduces dramatically the accuracy.
It is an object of the present invention to provide an improved apparatus that allows to quickly and reliably detect a preamble used, for example, in an FSK or DFSK communication and to estimate and/or remove the offset.
It is another object of the present invention to provide an improved receiver comprising such an improved apparatus.